The present invention relates to an optical communication system and, in particular, to a dynamic bandwidth allocation method which supports multiple modulation schemes, an OLT, and a PON system.
A PON (Passive Optical Network) system generally employs TDMA (Time Division Multiple Access) when transmitting uplink signals from plural ONUs (Optical Network Units, i.e., customer premises equipment) to an OLT (Optical Line Terminal, i.e., station-side equipment). The OLT performs a DBA (Dynamic Bandwidth Allocation) process for allocating a transmission start time and a transmission duration, i.e., a transmission timing of each ONU, based on a transmission request from each ONU, i.e., the data size stored in the ONU. Then, the OLT notifies each ONU of that result. In this manner, it is possible for the OLT to receive the uplink signals transmitted as burst signals from the respective ONUs without collision. In this case, in order to avoid collision between the burst signals from the respective ONUs, it is considered to ensure a certain time interval (guard time). The message format and the sequence of notifications between the OLT and the ONU are defined in IEEE802.3ah Standards as MPCP (Multi-Point Control Protocol). However, an algorithm for dynamic bandwidth allocation is not defined in Standards.
As one technique for effectively using an optical transmission medium in the PON system, a method can be considered which allows plural kinds of modulation schemes to coexist. For example, in some cases the distance from the OLT to each ONU is not constant. In general, an optical signal degrades more largely as the propagation distance becomes longer. Therefore, when 16QAM (Quadrature Amplitude Modulation) is used for an ONU at a relatively short distance from the OLT and QPSK (Quadrature Phase Shift Keying) is used for an ONU at a long distance, the efficiency of the use of the entire transmission medium can be improved.
In the aforementioned PON system including plural kinds of modulation schemes coexisting, the OLT has to switch a modulation scheme set in a modulation-scheme variable burst-mode optical transmitter/receiver provided therein, to correspond to the modulation scheme employed for each ONU in accordance with a timing of transmission of an uplink signal from that ONU. That is, the aforementioned guard time is a time which also includes a time required for switching the modulation scheme (OLT is known).
An exemplary background technique in this art is JP-T-2010-509812.